1. Field of the Invention
The present invention relates to a reforming apparatus for fuel cells, a fuel cell and an operation method of the fuel cell.
2. Description of the Related Art
In a fuel cell system in which a relatively easily handled fuel such as natural gas and methanol is utilized, it is necessary to convert the fuel into a gas mainly composed of hydrogen (namely, the so-called “reforming” is conducted), and then feed the gas into an anode of the fuel cell body. As a method for performing this reforming, a method is in common use in which a fuel gas and steam are reacted with each other. This method requires a high purity steam, necessitating an apparatus for producing high purity water and an apparatus for producing steam, including utilities for these apparatuses.
Additionally, examples of the reforming method include a method in which a fuel gas is partially oxidized with air. This method has the advantage that the facility configuration is simpler than that for the steam reforming method and startup is instantaneous, but has the drawback that energy loss is high and contamination with nitrogen lowers the efficiency.
Additionally, a method is known in which a fuel gas and carbon dioxide are reacted with each other to generate hydrogen and carbon monoxide (namely, the so-called carbon dioxide reforming). However, in low temperature fuel cells (fuel cells for which the operation temperature is 230° C. or less) such as polymer electrolyte fuel cells, if the gas fed into the anode contains carbon monoxide, the platinum based electrode catalyst in the anode is deteriorated to inhibit the anode reaction of hydrogen, and it is therefore necessary to remove the carbon monoxide. Thus, the “carbon dioxide reforming” high in generation of carbon monoxide is not suitable as the reforming method for the low temperature fuel cells such as polymer electrolyte fuel cells.
On the contrary, in solid oxide fuel cells which are among high temperature fuel cells (fuel cells for which the operation temperature is 500° C. or more), carbon monoxide does not inhibit the anode reaction of hydrogen, but carbon monoxide itself carries out an anode reaction. Consequently, in the case of the solid oxide fuel cell, it is advantageous to adopt the “carbon dioxide reforming” which is high in the generation of carbon monoxide.
In these years, on the other hand, the use of dimethyl ether (hereinafter abbreviated as “DME”) as fuel gas for fuel cells has been investigated. Dimethyl ether is a liquefied gas and hence has advantages that dimethyl ether has a high energy density (19.2 MJ/l), has little toxicity, is transferable in a condition of being contained in a steel cylinder, and is more easily reformed than propane, a similar liquefied gas, because of containing oxygen in the molecular structure thereof.
In view of the above described circumstances, for the purpose of actualizing a highly efficient, compact solid oxide fuel cell, it is probably suitable to conduct the carbon dioxide reforming by using dimethly ether as fuel gas; however, there is a problem of preventing the carbon deposition at the time of reforming.
Incidentally, the aforementioned reforming methods are described in “Trend in Research of Fuel for Fuel Cell,” (in “Development of Fuel Cell Vehicles and Related Materials,” Chapter 3), Ken Nozaki, C. M. C. Publishing Co., December, 2002. Additionally, JP2002-505511A (WO99/44253) describes the use of dimethyl ether as fuel gas for a polymer electrolyte fuel cell.
As described above, when steam reforming is conducted, peripheral equipment for the reforming apparatus or the fuel cell becomes complex and the startup time is longer. In the case of the partial oxidation, the efficiency is lowered. Additionally, nitrogen gas is used at the time of shutdown or emergency shutdown, and hence the peripheral equipment becomes complex. The present invention takes as its first object the solution of these issues, namely, the shortening of the startup time of the fuel cell, and the simplification of the peripheral equipment and control.
A second object of the present invention is the prevention of the carbon deposition in the reforming reactor or in the anode chamber in the case of the operation method of the fuel cell configured in such a way that the anode reaction of carbon monoxide as well as the anode reaction of hydrogen occurs, and either there is fed into the anode chamber a gas containing hydrogen and carbon monoxide generated by reacting dimethyl ether and carbon dioxide with each other in the reforming reactor, or there are fed into the anode chamber dimethyl ether and carbon dioxide without the intermediary of the reforming reactor.